The organic EL device, particularly low-molecular weight organic EL (OLED for short hereinafter) device, has greatly improved in characteristic properties since development by Eastman Kodak Company in the structure of organic layers, such as extremely thin layers and multiple layers for separate functions and substantially reduced driving voltage. (See, for example, Applied Physics Letters, U.S., 1987, vol. 51, p. 913 to 915.)
The recent organic EL device, such as the one based on a polymeric luminescent material (PLED for short hereinafter), which has been developed by Cambridge University, is comparable to the conventional OLED device. (See, for example, Nature, England, 1990, vol. 347, p. 539 to 541.)
On the other hand, it has been reported that the OLED device improves in initial characteristic properties (such as reduced driving voltage and increased light emitting efficiency) and lifetime, if it has a copper phthalocyanine (CuPC) layer as the hole injection layer. (See, for example, Applied Physics Letters, U.S., 1996, vol. 69, p. 2160 to 2162.)
It has also been reported that the PLED device produces the same effect as the OLED device if it has a hole transporting layer (buffer layer) formed from a polyaniline-based material or a polythiophene-based material. (For the former, refer to Nature, England, 1992, vol. 357, p. 477 to 479, and Applied Physics Letters, U.S., 1994, vol 64, p. 1245 to 127. For the latter refer to Applied Physics Letters, U.S., 1998, vol. 72, p. 2660 to 2662.)
In addition, it has been found that an electron injection layer adjacent to the cathode improves the initial characteristic properties. The electron injection layer is formed from any of metal oxide, metal halide, and metal complex, which were reported respectively in IEEE Transaction on Electron Devices, U.S., 1997, vol. 44, p. 1245 to 1248; Applied Physics Letters, U.S., 1997, vol. 70, p. 152 to 154; and Japanese Journal of Applied Physics, 1999, vol. 38, p. L1348 to 1350. These findings led to the common use of charge injection layer and buffer layer.
It has recently been found that the EL device exhibits outstanding characteristic properties if it has a hole injection layer formed from a charge transporting varnish in the form of organic solvent solution of low-molecular weight oligoaniline. (See, for example, Japanese Patent Laid-open No. 2002-151272.)
Unfortunately, CuPC as a common material for the hole injection layer in the OLED device has the disadvantage of greatly deteriorating the characteristic properties when it is added to other organic layers no matter how small its quantity may be because of its large surface irregularities. Moreover, polyaniline- and polythiophene-based materials, which are currently used for the PLED device, involves problems arising from the fact that they contain water as a solvent which accelerates device deterioration, they are limited in solvent selection, and they are limited in method of uniform film forming because of their low solubility and tendency toward aggregation. Even the charge transporting varnish in the form of organic solvent solution containing a highly soluble low-molecular weight oligoaniline-based material is limited in the kind of electron accepting dopant that can be used because most electron accepting dopants are poor in heat resistance and amorphousness. A problem common to the charge transporting varnish containing a low-molecular weight charge transporting substance and a charge accepting dopant substance, particularly the varnish containing a crystalline substance, is that they usually present difficulties in forming a perfectly flat film.